Forest Surveying

Forest Surveying

Forest Engineering is the application of scientific, economic, social, and practical knowledge in order to invent, design, build, maintain, and improve structures, machines, devices, systems, materials and processes concerning to Forest. The discipline of engineering is extremely broad, and encompasses a range of more specialized fields of engineering, each with a more specific emphasis on particular areas of applied science, technology and types of application.

Survey, scope and types of surveying,

To Survey, as defined in Webster’s Dictionary, is

“to determine and delineate the form, extent, position, etc., of a tract of land, by taking linear and/or angular measurements and by applying the principles of geometry and trigonometry.”

As such, Surveying may be defined as the art of making such observations and measurements—linear and/or angular—as will determine the relative positions of objects on the earth’s surface, an representing the . same on a plan or a map, drawn to a fixed scale.

If the scale is large enough to show even minor details, the drawing is called a play ; if the scale is small and the drawing covets a large area, showing a few details only, it is called a map.

Surveying. in short, may be regarded as the art and science of map making. The process of surveying involves both field work of faking measurements and office work of computation and drawing. The drawing generally shows horizontal distances ; sometimes vertical distances may also be represented, approximately, by means of contours, hachures, etc. Levelling deals with the determination of relative heights or elevations of various points or objects on the earth’s surface, above a fixed reference point called datum.

It deals with measurements in a vertical plane. Surveying and levelling are generally considered as distinct operations ; comprehensively speaking, levelling is only a branch of Surveying– the latter including the former.

Objects and Scope of Forest Surveys:

Knowledge of theory and practice of surveying and map reading are an important part of a forester’s training. Charged with the responsibility of protection, ‘ management and administration of a forest estate, a forester is often required to map out fire-burnt areas ; demarcate or check forest boundaries ; lay out felling coupes prepare plantation maps and stock maps ; detect and rectify encroachments and illicit possessions ; prepare plans of areas to be cleared, or fenced or planted ; align extraction roads and paths, and for a variety of other purposes connected with his duties. He should be able to prepare and interpret maps and plans correctly.

Foresters are generally required to carry out the following types of survey :

  • Chain Survey
  • Chain and Compass Survey
  • Plane table Survey
  • Levelling.

CLASSIFICATIONS (DIVISIONS/BRANCHES) OF SURVEYING

(a) Primary classification (or Division) -Surveying may be classified into two main categories, viz.,

(i) Plane Surveying and

(ii) Geodetic or Trigonometrical Surveying.

In Plane Surveying, it is assumed that the surface of the earth is plane, and not curved as it actually is ; in other words, curvature of the earth is ignored. Upto about 250 sq. km area, the sphericaly of the earth introduces only a. negligible error, and the surface can be assumed as plane for ordinary surveys. Consequently, the line joining any two points on the earth’s surface is reckoned as straight and not curved, and the angle between any two straight lines on the surface as plane and not spherical. For ordinary surveys where high degree of precision is not required, such as forest surveys,

In Geodetic Surveying, the curvature of the earth’s surface is taken into account. A line connecting any two points on the surface is considered as an arc and not a straight line, and all angles of triangles as spherical and not plane. It, therefore, involves knowledge of spherical geometry and trigonometry. Geodetic Surveying or Geodesy is indicated where area involved is large and great precision and accuracy is desired ; it requires refined methods of observation and computations, for determination of the absolute position on the earth’s surface of a series of points which serve as controls for all other surveys. Such surveys, called Geodetic or Great Trigonometrical Surveys, are usually of a national character and carried out through Govt. agencies ; in India, by the Survey of India Department.

Secondary classification of Surveying Surveys may further be classified and sub-classified in a variety of ways :—

Classification based on the nature of the Field of Surveys

(a) Land Surveys (b) Marine or Navigation Surveys (c) Astronomical Surveys

Land Surveys are further classified as;–

(i) Topographical Surveys, carried out for delineating the natural physical features of the earth’s surface, such as hills, valleys, ridges, rivers etc

(ii) Cadastral Surveys in which additional details such as boundaries- of fields, paths and other such artificial details are also determined. These are field to field surveys conducted by Govt. in a Revenue or a village, for settlement of land revenue, recording existing rents and fixing fair rents. The scales usually adopted are 1/4000, 1/2000 and 1/1000 corresponding to old scales of 16″= 1 mile for villages. 32 inches or 64 inches to a mile for towns.

(iii) City Surveys for laying out of roads, streets, drainage channels, water pipes, sewers

(iv) Engineering Surveys for designing and construction of engineering projects, such as canals, roads, railways., etc.

Classification based on Methods employed in Surveying :

(I) Traverse Surveys (ii) Triangulation Surveys

Classification based on Instruments Used :

(I) Chain Surveys (ii) Compass Surveys (iii) Plane-table Surveys (iv) Theodolite Surveys (v) Photogr6hic Surveys (vi) Technometric Surveys.

Classification based on Objects of Surveys

(i) Geological & Soil Surveys ,(ii) Mineral Surveys (iii) Archaeological Surveys (iv) Military Surveys.

Two Great Principles of executing a Survey —

First principle is to work from the whole to the part and not from part to the whole. The essence of this principle is to establish a system control points with high standard of accuracy, by triangulation or a traverse ; however between them the work may be done by less accurate and, consequently, less expensive methods. For instance, in triangulation survey the main triangles should be as large sized (hence as few) as possible, and surveyed with greatest accuracy ; these will be further sub-divided into smaller minor or secondary triangles which will generally be measured with less rigid methods , and less elaborate instruments. The idea of working in this way if to avoid the accumulation of error, which would have occurred ii we proceeded from part to the whole, and also to localize minor errors, if any. This principle also applies to such operations as levelling. Thus, in contouring on a large scale it is advisable to establish a system of bench marks with accurate precise level. The actual survey of the contours or form lines can be done by using an Abney Level or Indian Clinometer, or a similar instrument of lesser accuracy. In the method of traverse survey by chain angles, minor errors get magnified progressively ; this method is opposed to the above principle and should be resorted to only in exceptional circumstances.

Second principle is to ascertain the dimensions of main lines and positions of the new points by at least two independent processes. The points may be fixed by linear, angular or both linear and angular measurements. 

In chain survey, measurement and plotting of the main lines and station points are checked by means of check or tie lines.

Methods of Locating a Point 

Relative positions of points are located by measurements from at least two points of reference whose positions are correctly known. Let A & B be two known points on the ground and the plan, and a point C is to be located w.r.t. A & B on paper. This can be done by any of the following methods ; . 

  1. Measure AD or BD and perpendicular CD : Fig. 1.1 (a).

2. Measure AC and BC : Fig. 1.1 (b).

3. Measure angle BAC and distance AC ; or angle CBA and distance BC : Fig. 1.1 (c).

4. Measure angle CBA and distance AC or angle CAB and distance BC : Fig. 1.1 (d).

5. Measure angles CBA and CAB : Fig. 1.1 (e).

Applications of survey : 

The surveying methods can be used for the design and constniction of the following items:

1. Farm road

2. Irrigation and drainage channels

3. Planning of farm building and service structures

4. Soil conservation structures

5. Preparation of field map

6. Laying of railways and bridges culverts etc.

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